The invention relates to a process for galvanic depostion of a dispersion coating, having a metallic matrix and fine grained particles of hard material uniformly distributed therein, on the cylindrical or slightly conical, metallic inner face of a cathodically connected workpiece by continuous feed of a circulating electrolyte containing metal ions and suspended, fine grained hard particles. The invention relates also to the use of the process and a device for performing the said process.
In many cases the use of metals in industrial fields requires an improvement in surface properties, in particular wear resistance, hardness and sliding properties. Many of the applications open to aluminum in the automobile and machine construction industries can be realized only in combination with hard and wear resistant coatings. The galvanic deposition with a metal layer and simultaneous incorporation of hard particles in the layer to give a dispersion-type coating provides a simple and, for many wear problems, suitable possibility for improving the surface and its mechanical properties.
Such dispersion coatings, mostly of the nickel/silicon-carbide system, exhibit many combinations of properties as a result of varying the matrix material, particulate material, particle size and distribution. The production of galvanically deposited dispersion coatings has been known for some decades now. The British Pat. No. 860 291 for example describes a coating process wherein an electrolyte is fed to a tank via a supply pipe above the tank, is removed again from the bottom of the tank and fed to the electrolyte circuit. The body which is to be coated on the surface rotates in the electrolyte. Parts of the surface which are not to be galvanically coated must be coated e.g. with lacquer or paint before immersion in the electrolyte. After the galvanic coating has been deposited, a considerable amount of subsequent treatment is necessary because the coating is very rough. Another problem is that incorporation of the solid particles in the precipitated metal varies very markedly, according to the conditions of electrolyte flow; uniform coatings are not possible via British Pat. No. 860 291. If internal faces of a hollow body are to be coated, the blisters formed during immersion produce a further detrimental effect on the coating process.
In the technical journal "Oberflachentechnik" (1975) pp 42-52 it has been pointed out that the movement of the electrolyte has a large effect on the rate of incorporation of hard particles in the metal being deposited. It is proposed there to effect movement of the electrolyte by blowing air into the bath, rotating the electrolyte or stirring it mechanically. The resultant movement of the electrolyte is intended to enable the solid particles to be transported with the electrolyte above the workpiece so that, under the influence of gravity, they can settle on the surface of the workpiece and be bonded there by the metal coating. It has, however, been found that all these methods are unsuitable in as much as they lead to inhomogeneity or concentration differences of hard suspended particles in the electrolyte and thus also result in irregular rates of incorporation of the dispersoid. Changes in turbulence along the workpiece to be coated always yield irregular deposition of the solid component.
Described in the German patent publication DE-OS No. 31 42 739 is a process and device for depositing a dispersion coating on hollow workpieces without suffering from the above mentioned disadvantages. To this end the hollow, in particular a cylindrical or conical workpiece is used as part of the electrolyte container and electrolyte is fed to the inner face via a moving supply line i.e. the means of feeding the suspension type electrolyte is moved along the surface of the workpiece to be coated.